Propagating electrical signals have an electrical field component and a magnetic field component. Electrical signals cause circuit components to radiate a portion of the spectral energy of the propagating signal causing electromagnetic interference. Electromagnetic interference is the generation of undesired electrical signals in electronic system circuitry because of the unintentional coupling of impinging electromagnetic field energy. Circuit elements are effective in radiating spectral components which have wavelengths similar to the radiating element dimensions. Long circuit elements will be more effective in radiating low frequency noise and short circuit elements will be more effective in radiating high frequency noise. These circuit elements behave like antennae which are designed for the transmission of the radiating wavelengths.
Integrated circuits which have output drivers that create pulses with high amounts of spectral energy are more likely than low power drivers to cause electromagnetic interference because of a mismatch between the driver and the line impedance, and the resistance to instantaneous signal propagation imposed by the parasitics of the conductor. These disturbances in the electromagnetic field result in reflections of the signal energy at the points where the variation occurred. If the signal is not absorbed by the load at the end of the conductor length because of unmatched impedances or lack of line termination, the unabsorbed energy will be reflected back towards the source giving rise to radiated emissions. Proper termination and controlled impedance interconnections can reduce radiated noise.
The coupling of signal energy from an active signal net onto another signal net is called crosstalk. Crosstalk is within-system electromagnetic interference as opposed to electromagnetic interference from a distant source. Crosstalk is proportional to the length of the net parallelism and the characteristic impedance level, and inversely proportional to the spacing between signal nets. Proper interconnect layout design can reduce the incidence of crosstalk. Strong sources of low impedance, magnetic field rich electromagnetic interference are relatively high current and relatively low voltage components. Magnetic fields possessing high intensity can induce spurious current flow in other system components. Noise radiated from within a system can interfere with system performance by coupling with other system elements, not just adjacent conductor nets, as another form of within system electromagnetic interference.
Because electronic systems are becoming smaller and the density of electrical components increasing, the dimensions of the average circuit element is decreasing favoring the radiation of higher frequency signals. The increasing operating frequency in these electrical systems results in increasing high frequency electromagnetic interference. Electromagnetic interference can come from electrical systems distant from a receiving circuit, or the source of the noise can come from a circuit within the same system (crosstalk or near source radiated emission coupling). The effect of all these sources of noise is to degrade the performance or to induce errors in the systems. The prevalence of high frequency systems and portable electronics is creating a very complex spectral environment for the operation of sensitive electrical systems.
The electromagnetic interference shielding of electronic component assemblies has taken many forms. Sensitive or radiating devices may be covered with a lid and/or enclosure which is connected to ground potential in the process of securing the cover in place. Shielding close to the source, where the field intensity is the highest, requires greater shield efficiency to contain the field. It is common to shield the sensitive, electromagnetic interference receiving component or even the entire circuit board. Polymer thick film conductor materials, such as a screen-printable copper filled epoxy paste, are sometimes used to form a shield. Individual ferrite components may be placed on device pins or in series within a circuit to attenuate unwanted noise. A ferrite component may be used with a capacitor in order to form a low frequency inductance-capacitance band pass filter. Many enclosed systems powered by external alternating current are shielded from electromagnetic interference by the incorporation of internal shields which are connected to ground potential. A metal cabinet housing which encloses the system may be designed to function as a shield. Disadvantages of metal housings are that they are often expensive, heavy, and difficult to make in complex shapes. The inside of a molded plastic housing may be coated with a thin metal film through vacuum metalization but this process often yields a brittle less flexible shield. Another method is to coat the enclosure with a thin film of a conductor using a metal-filled paint. A metal-filled plastic may also be used to form the housing.
U.S. Pat. No. 4,012,089 (Ward) discloses an electronic instrument enclosure using a molded, laminated plastic enclosure having an outer shell made of a thermoplastic composition which has desirable physical and electrical characteristics (see FIG. 1). An inner plastic shell is nested in the outer shell with a stiffening filler material sandwiched between. Heat pipes may be imbedded in the shells to provide cooling for the enclosed electrical components and inserts for the mounting of slides and guidepins can be molded in the inner shell for ease in mounting hardware for slidable drawers. The inside walls of the inner shell may be vacuum metalized or plated to provide radio frequency shielding.
U.S. Pat. No. 4,227,037 (Layton) discloses a container having complementary upper and lower portions (see FIG. 1). Each portion is adapted to mate with and engage to define an enclosed inner chamber. The upper and lower portions each have laminated outer casings. A non-metallic, electrically conductive, inner layer is integrally molded with and bonded between the outer non-metallic reinforcing layers to shield the inner chamber from electromagnetic and radio frequency interference.
U.S. Pat. No. 4,678,716 (Tzeng) discloses an electrically conductive particle for use as a conductive filler in a resin matrix suitable for electromagnetic shielding use in gaskets (see FIG. 1). The particle comprises an inner core of an aluminum silicon alloy having from 5% to 20% by weight of silicon, an intermediate layer of a metal selected from the group consisting of mercury, palladium, copper, chromium, platinum, gold, nickel, tin, and zinc, and an outer layer of a highly electrically conductive metal.
U.S. Pat. No. 4,739,453 (Kurokawa) discloses a shielding apparatus for shielding electric circuitry mounted on a printed circuit board against interfering electric waves (see FIG. 1). The shielding apparatus comprises a multi-layer printed circuit board having a circuit trace printed on a middle layer. A first circuitry block is mounted on an outer surface of the multi-layer printed circuit board and is electrically connected to the circuit trace so that the first circuitry block is connected to one other circuitry block mounted on the multi-layer printed circuit board via the circuit trace. A metallic plate covers the first circuitry block to isolate the first circuitry block from the other circuitry block.
U.S. Pat. No. 4,831,498 (Baba) discloses a shield structure which is mounted on a circuit board. The shield structure comprises conductive pattern members, conductive box members, rib means, screw-fastening means, and conductive through-holes. A first conductive pattern member is formed on a top surface and a second conductive pattern member is formed on a bottom surface of the circuit board. The first conductive pattern member is formed along a shield-requiring region on the top surface of the circuit board and the second conductive pattern member is formed along a shield-requiring region on the bottom surface of the circuit board. A first conductive box member has an open side and defines a shielded space enclosing the shield-requiring region on the top surface of the circuit board. A second conductive box member has an open side and defines a shielded space enclosing the shield-requiring region on the bottom surface of the circuit board. A first conductive rib means is formed on an edge defining the open side of the first conductive box member and is connected to the first conductive pattern member. A second conductive rib means is formed on an edge defining the open side of the second conductive box member and is connected to the second conductive pattern member. A screw-fastening means secures the first conductive box member and the second conductive box member in pressure contact with the circuit board and for causing the first conductive rib means to bite into the first conductive pattern member and for causing the second conductive rib means to bite into the second conductive pattern member. Conductive through-holes are provided with conductive linings formed on the top surface and bottom surface of the circuit board and are electrically connected to the first conductive pattern member and the second conductive pattern member.
U.S. Pat. No. 4,857,668 (Buonanno '668) discloses a multi-function gasket for electrical apparatus which generate or are adversely affected by electromagnetic and radio frequency interference (EMI/RFI). The multi-function gasket comprises a continuous, molded, resilient foam core having a sealed outer boundary layer. A flexible, electrically conductive and substantially abrasion resistant sheath externally surrounds the foam core and bonds to the boundary layer. The foam fills the interior of the sheath. A means for mounting the gasket is provided whereby an apparatus may be sealed against EMI/RFI leakage, noise emission and environmental infiltration through perimeter gaps of electrically conductive doors, access panels by the actions and interactions of the sheath, the foam core and the boundary layer. The flexible sheath is continuously pressed by the resilient foam core into positive engagement with conducive surfaces between which the gasket may be mounted forming a continuous electrical path enclosing the foam core extending continuously across the gaps and preventing EMI/RFI leakage through the gaps. The boundary layer prevents noise emission and environmental infiltration across the gaps and the sheath protects the boundary layer against damage from abrasion.
U.S. Pat. No. 4,967,315 (Schelhorn) discloses a shielded RF package having a ceramic seal ring and a ceramic circuit supporting substrate positioned between metallic base and lid elements. The ceramic elements are metallized over portions of three of their surfaces to permit conductive continuity between the seal ring, the substrate, and the metallic lid and base of the package providing integral shielding and ground for the microwave components mounted in the package. The surface metallization of the ceramic components is patterned to produce electrical isolation at the input/output ports of the package, but provide an integral ground connection between the package lid and the metallic package base. The shielded RF package comprises a ceramic circuit board substrate having an edge and broad flat upper and lower surfaces for supporting electrical components above the flat upper surface. The substrate includes metallization traces on the upper surface for defining interconnections of the components. The traces extend to a region near the edge for defining an input or an output conductor. The substrate is metallized on the upper surface around the periphery near the edge except in an open region adjacent the input and output conductor to form an upper ground conductor. The substrate is also metallized around the periphery of the broad lower surface near the edge to define a lower conductor. The substrate is also metallized on the edge for interconnecting the upper ground conductor and the lower conductor. A ceramic seal ring defines horizontal upper and lower surfaces and inner and outer vertical surfaces. The seal ring is metallized on the horizontal upper surface and on the outer vertical surface. The seal ring is also metallized on the horizontal lower surface except in an open region to form a continuous conductive path between the metallization on the horizontal upper and lower surfaces of the seal ring by way of the metallization on the outer vertical surface. The seal ring is mounted upon the substrate with the unmetallized open regions of the substrate and ring in registry and with the metallization on the horizontal lower surface of the seal ring in electrical contact with the metallization on the upper surface around the principal portion of the periphery. The seal ring is sealed in place with glass material in the unmetallized regions. A metallic lid is supported by the metallization of the horizontal upper surface of the seal ring and sealed thereto by a first reflowed metal preform. A metallic base supports the ceramic circuit board substrate and is sealed to the periphery of the metallized portion of the lower surface of the ceramic circuit board substrate by a second metal preform.
U.S. Pat. No. 5,107,404 (Tam) discloses a circuit board assembly for a cellular telephone system. The circuit board assembly comprises a printed circuit board having multiple layers including an interior ground plane and an interior signal plane and having a number of electronic components and a number of ground plane lines and a number of signal plane lines on an outer surface which is opposite the electronic components. The ground plane is interrupted in defined regions to permit the signal plane lines to tunnel underneath the ground plane lines. The printed circuit board has a number of holes which are plated through to interconnect the interior ground plane with the ground plane lines. The circuit board assembly has a housing for enclosing the printed circuit board. The housing comprises a frame having an outer wall surrounding the periphery of the printed circuit board and defining an interior space. The frame includes a number of interior walls spanning the interior space and mounting means mounting the printed circuit board to the frame to span the interior space defined by the frame. The ground plane lines on the printed circuit board are in alignment with and electrically engage the adjacent edge of the interior walls of the frame. The housing comprises first and second cover plates spanning the interior space defined by the outer wall of the frame on opposite sides of the printed circuit board to enclose the printed circuit board within a housing chamber defined by the outer wall of the frame and the first and second cover plates. One of the cover plates has interiorly extending walls corresponding to and in alignment with the interior walls of the frame on the opposite side of the printed circuit board and electrically engaging the ground plane lines on the printed circuit board. The interior walls form a number of sub-chambers within the housing chamber. The sub-chambers are arranged to isolate respective ones of the electronic components on the printed circuit board. Securing means directly connect the first and second cover plates to each other independently of the printed circuit board and the frame such that the first and second cover plates are pressed against the frame to secure the frame between the first and second cover plates.
U.S. Pat. No. 5,202,536 (Buonanno '536) discloses a seal for blocking propagation of electromagnetic energy through a gap between bodies having conductive surfaces adjacent the gap. The seal comprises an elongated core element defining a resiliently compressible cross section. A flexible elongated conductive sheath portion is attached to the core element at a surface of the core element exposed to the conductive surfaces of the bodies. The conductive sheath portion extends part way around the cross section and defining ends which are spaced on the core element and non-overlapping. One additional flexible and elongated sheath portion is attached to the core element and extends between the ends of the conductive sheath portions. The conductive sheath portion and the additional sheath portion together extend fully around the cross section of the core element. The conductive sheath portion defines a conductive exterior surface of the seal extending around a first part of the cross section of the seal and the additional sheath portion defining a nonconductive exterior surface of the seal extending around a second part of the cross section of the seal. The additional sheath portion is lapped with one of the spaced, non-overlapping ends of the conductive sheath portion. The conductive sheath portion and the additional sheath portion respectively define electromagnetic and environmental barriers bridging across the gap.
U.S. Pat. No. 5,548,121 (Balmer et al.) discloses an electronically shielded solid state charged particle detector system. The shielded solid state charged particle detector system comprises a conductive detector housing having a detector entrance window for receiving charged particles. A charged particle detector has an active surface disposed within the conductive detector housing, the active surface facing the detector entrance window for providing electrical signals representative of the received charged particles when the received charged particles are applied to the active surface. A conductive layer is disposed upon the active surface and is electrically coupled to the conductive detector housing to provide a continuous conductive electrical shield surrounding the active surface.
U.S. Pat. No. 5,566,055 (Salvi, Jr.) discloses an EMI/RFI shielded cover assembly for an electronics enclosure. The cover assembly comprises a cover plate having a major surface area and interface locations for sealing against the electronics enclosure. A shielding compound is disposed in a layer across the major surface area and the interface locations on the cover plate. The shielding compound provides EMI/RFI shielding for the major surface area, and resilient gasket-like response at the interface locations to enable the cover assembly to be sealed to the electronics enclosure.
U.S. Pat. No. 5,594,200 (Ramsey) discloses an electromagnetic isolation chamber containing a volume of space electromagnetically isolated from the surrounding environment. The electromagnetic isolation chamber comprises a wall enclosing the volume, the wall having first electromagnetic shielding surrounding the volume and having an aperture wherein the first shielding is disrupted. A flexible, conductive second electromagnetic shielding is disposed within the chamber and covers the aperture in the wall and is conductively and shieldingly attached to the first electromagnetic shielding of the wall around the periphery of the aperture. The second shielding is deformable and of sufficient size to enclose objects which may be inserted into the chamber through the aperture and to permit tactile feel and manipulation of the objects within the chamber so that continuity of electromagnetic isolation of the volume is maintained through the insertion.
U.S. Pat. No. 5,712,449 (Miska et al.) discloses a gasket for blocking electromagnetic radiation between two electrically conductive bodies. The gasket comprises a compressible core generally shaped as a sheet extending along part of the length and width of the body. An electrically conductive surface material is disposed on opposite faces of the core for bearing against the conductive bodies. A number of electrically conductive connections at spaced positions electrically couples between the electrically conductive surface material on the opposite faces.
U.S. Pat. No. 5,717,577 (Mendolia et al.) discloses an apparatus for shielding electromagnetic emissions created by electronic components and circuitry. The apparatus comprises a printed circuit board for mounting the electronic components. The printed circuit board includes a ground plane and a ground pad ring located on a top surface of the printed circuit board surrounding the electronic components. A means for electrically connecting the ground pad ring to the ground plane is provided. A shield can has a lip extending along a perimeter which is aligned with the ground pad ring. A gasket is constructed of a semi-lossy conductive material positioned between the lip along the perimeter of the shield can and the ground pad ring to provide a conductive seal and to attenuate electromagnetic emissions within the shield can.
U.S. Pat. No. 5,748,455 (Phillips et al.) discloses an electromagnetic shield for shielding an electrical circuit on a circuit board which is a conductive strip extending around the electrical circuit. The electromagnetic shield comprises a face, a side wall extending around the face, and a series of spring contacts extending from the side wall for electrical contact with the conductive strip extending around the circuit board. The spring contacts are flexible and movable up and down relative to the face of the shield and form an integral one-piece construction with the side wall. Each spring contact includes a first tab that projects from the plane of the sidewall and a finger that extends from the first tab and flexes about a fold line extending between the first tab and the finger. The shield is installed between the circuit board and a housing. The respective first tabs project outwardly from the sidewall and include upper edges that are engaged by the housing causing the shield to be pressed towards the circuit board where the spring contacts engage the conductive strip.
U.S. Pat. No. 5,763,824 (King et al.) discloses a shielding cover in combination with an electrical assembly. The electrical assembly has an electrical ground, an electronic component electrically connected to the assembly, and a conductive frame which is disposed about the electronic component and which is electrically connected to the ground. The conductive frame has a mounting surface. The shielding cover comprises a lid and an electrically conductive adhesive disposed between the conductive frame and the lid. The electrically conductive adhesive has a substrate having passageways through the substrate defined by a number of internal surfaces having disposed a layer of conductive metal. The passageways are partially filled with a nonconductive adhesive resin.
U.S. Pat. No. 5,811,050 (Gabower) discloses a method for forming an EMI shield from polymeric material. The method comprises thermoforming sheets of thermoformable polymeric material into desired shapes. The thermoforming process comprises heating a thin sheet of thermoplastic polymer, drawing the heated sheet into an open mold or onto a die, cooling the formed sheet, removing the formed sheet from the mold or die, and applying electrically conductive metallic material to selected surfaces of the thermoformed polymeric shapes by vacuum deposition means.
U.S. Pat. No. 6,016,083 (Satoh) discloses an electronic circuit apparatus for suppressing electromagnetic radiation. The apparatus comprises an electronic circuit mounted on a top surface of a printed-circuit board. An input-output terminal is mounted on the printed-circuit board. A shield-case for suppressing electromagnetic radiation from the electronic circuit is included. A conductor-piece, allocated between the electronic circuit and the input-output terminal is provided connecting a first ground-pattern stuck to a bottom surface of the printed-circuit with the shield-case. A means for connecting the first ground-pattern with the input-output terminal via the conductor-piece is included. The connecting means is composed of a second ground-pattern stuck to the bottom surface of the printed-circuit board near the input-output terminal and a metallic-connecting-piece for connecting the second ground-pattern with the shield-case.
A common feature of these and other prior art electromagnetic interference shielding methods is that these methods focus on the enclosure and not on the radiating source. A break in the shield will form an aperture through which radiation can escape and thus great care must be taken to use conductive gaskets to seal access areas. In addition, a break in the shield may require that the entire electroninc device, or significant parts of it, be discarded as waste at a considerable expense. Conventional metalizing methods such as using conductive metal-bearing paints, vacuum plating of aluminum, physical vapor deposition of aluminum or other metals, plating on plastics, laminated metalizing methods, and using woven and coated fibers generally fail due to limited ductility, flexibility, and thermoformability.